Plan 9 on Android — replacing the Java stack on Android fans with Inferno. Inferno is the Plan 9 operating system originally from Bell Labs.
SmartOS — Joyent-created open source operating system built for virtualization. (via Nelson Minar)
libtcod — open source library for creating Rogue-like games. (via Nelson Minar)
Wikipedia Miner — toolkit for working with semantics in Wikipedia pages, e.g. find the connective topics that link two chosen topics. (via Alyona Medelyan)